Klein four-group

In mathematics, the Klein four-group is an abelian group with four elements, in which each element is self-inverse (composing it with itself produces the identity) and in which composing any two of the three non-identity elements produces the third one. It can be described as the symmetry group of a non-square rectangle (with the three non-identity elements being horizontal reflection, vertical reflection and 180-degree rotation), as the group of bitwise exclusive or operations on two-bit binary values, or more abstractly as Z2 × Z2, the direct product of two copies of the cyclic group of order 2 by the Fundamental Theorem of Finitely Generated Abelian Groups. It was named Vierergruppe (German: [ˈfiːʁɐˌɡʁʊpə] ⓘ), meaning four-group) by Felix Klein in 1884. It is also called the Klein group, and is often symbolized by the letter V or as K4.

Not to be confused with Kleinian group, a discrete subgroup of PSL(2, C).
Algebraic structure → Group theory
Group theory
Basic notions
  • Subgroup
  • Normal subgroup
Group homomorphisms
Finite groups
  • Schur multiplier
Classification of finite simple groups
Modular groups
  • PSL(2, )
  • SL(2, )
Topological and Lie groups
  • Special linear SL(n)
  • Orthogonal O(n)
  • Special orthogonal SO(n)
  • Unitary U(n)
  • Special unitary SU(n)
  • Symplectic Sp(n)
Infinite dimensional Lie group
  • O(∞)
  • SU(∞)
  • Sp(∞)
Algebraic groups
  • Linear algebraic group
  • Reductive group

The Klein four-group, with four elements, is the smallest group that is not a cyclic group. There is only one other group of order four, up to isomorphism, the cyclic group of order 4. Both are abelian groups.

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